The present invention relates to an exhaust gas purifying apparatus, and in particular to such an exhaust gas purifying apparatus which includes a NOx adsorption/reduction catalyst that is able to resume its purifying capability in a short time, and which may be suitably used in an internal combustion engine of lean-burn combustion type or in-cylinder injection type.
One type of exhaust gas purifying device is known which includes a three way catalyst that provides an excellent purifying effect while the internal combustion engine is operating in a region around the stoichiometric ratio, and a lean-burn NOx catalyst that serves to remove NOx (nitrogen oxides) while the engine is operating in a lean-burn region. In this type of exhaust gas purifying device, the three way catalyst is located on the upstream side within an exhaust pipe of the engine, so that this catalyst can be rapidly activated so as to provide improved exhaust gas characteristics, in particular, immediately after the start of the engine. With the purifying device thus constructed, hydrocarbons and carbon monoxide contained in exhaust gases are consumed through exhaust gas purification of the three way catalyst, and therefore the concentrations of hydrocarbons and carbon monoxide have been reduced by the time when the exhaust gases reach the NOx catalyst located downstream of the three way catalyst. In this type of exhaust gas purifying device in which the NOx catalyst is located downstream of the three way catalyst, therefore, a NOx adsorption/reduction catalyst is generally used as the NOx catalyst, since little choice is left for using a NOx selective-reduction catalyst for removing NOx by reacting NOx with hydrocarbon, or the like. This NOx adsorption/reduction catalyst has a NOx adsorbent that adsorbs NOx in exhaust gases in the form of its oxidized product in an oxidizing atmosphere, and is adapted to reduce the oxidized product of NOx adsorbed on the NOx adsorbent in a reducing atmosphere, so that the NOx is dissolved or converted into a harmless nitrogen gas, while regenerating the NOx adsorbent. Even where this type of NOx catalyst is used, however, NOx is discharged into the atmosphere without being adsorbed onto the catalyst if the amount of NOx adsorbed on the catalyst reaches its saturation level.
In view of the above problem, it has been proposed in WO 93/07363 to switch the operating region of the internal combustion engine from a lean-burn region to a rich-burn region before the amount of adsorbed NOx reaches its saturation level, so as to form a reducing atmosphere around the catalyst, thereby to reduce the oxidized product of NOx in this reducing atmosphere.
However, an oxidized product of sulfur contained in the fuel, in addition to the oxidized product of NOx, is also deposited on the NOx catalyst. The thus deposited substances prevent deposition of NOx onto the catalyst, thereby reducing the NOx purifying or removing capability of the catalyst. Further, it is particularly difficult to reduce and remove the oxidized product of sulfur which has been deposited on the catalyst.
In view of the above problem, Japanese laid-open Patent Publication (Kokai) No. 6-88518 discloses a technique for resuming the purifying capability of the NOx catalyst, wherein the engine is controlled to operate in a rich-burn combustion region with the NOx catalyst being placed in a high-temperature environment, thereby to reduce and remove the oxidized product of sulfur.
In the technique disclosed in the above publication, however, the air-fuel ratio needs to be considerably reduced to provide a highly rich air/fuel mixture in order to reduce and remove the oxidized product of sulfur, and the fuel economy and exhaust-gas characteristics deteriorate during the regeneration process of this catalyst. Also, the output torque of the engine is undesirably changed due to changes in the air-fuel ratio upon transition from the normal operating region of the engine to the regeneration-process operating region, and upon return from the regeneration-process operating region to the normal operating region. In addition, it takes a considerably long time to reduce and remove the oxidized produce of sulfur through the regeneration process as described above. During this time, the above-described problems occur due to execution of the regeneration process.
It is therefore an object of the present invention to provide an exhaust gas purifying apparatus wherein the purifying capability of an NOx catalyst can be resumed in a relatively short time, by accelerating reduction of an oxidized product of sulfur deposited on the NOx catalyst.
To accomplish the above object, the present invention provides an exhaust gas purifying apparatus comprising: a three way catalyst provided in an exhaust pipe of an internal combustion engine; and a NOx adsorption/reduction catalyst provided in the exhaust pipe to be located downstream of the three way catalyst, wherein Ni or an oxide of Ni is added to the NOx adsorption/reduction catalyst.
In the exhaust gas purifying apparatus described above, when an oxidizing atmosphere is introduced into the exhaust pipe, a NOx adsorbent (catalyst source) which is present in the form of, for example, a metal oxide on the surface of the NOx catalyst reacts with NOx contained in exhaust gases, to thus form an oxidized product of NOx. The NOx adsorbent as described above also reacts with SO3 formed from SO2 that is generated by combustion of a sulfur content in the fuel, thereby to produce an oxidized product of sulfur. The thus formed oxidized products of NOx and sulfur are deposited on the NOx adsorption/reduction catalyst, and reduce its purifying capability.
When a reducing atmosphere is introduced into the exhaust pipe, SO2 contained in the exhaust gases is reduced on the three way catalyst and NOx catalyst, and hydrogen sulfide is formed. The hydrogen sulfide thus formed reacts with an oxide of nickel that is present on the NOx catalyst to which Ni or an nickel oxide is added, thereby to form a nickel sulfide. Also, the reactions to reduce an oxidized product of NOx and an oxidized product of sulfur take place.
It is to be noted that hydrogen sulfide is consumed as the above reducing reaction on the NOx catalyst to form nickel sulfide proceeds. To compensate for the hydrogen sulfide consumed in this manner, the above-described reduction to form hydrogen sulfide from SO2 is accelerated, and SO2 is consumed. To compensate for the SO2 thus consumed, the reaction that occurs on the NOx catalyst to reduce the oxidized product of sulfur is accelerated. Namely, the use of the NOx adsorption/reduction catalyst containing Ni results in an equilibrium state which causes rapid progress of the reaction in which the oxidized product of sulfur is converted into a NOx adsorbent. Consequently, the oxidized product of sulfur which induces reduction of the purifying capability of the NOx catalyst can be eliminated in a relatively short time.
The exhaust gas purifying apparatus as described above may further include temperature increasing means for increasing the temperature of the NOx adsorption/reduction catalyst. In this apparatus, the temperature of the NOx catalyst is increased during the reaction to reduce the oxidized product of sulfur, so that this reducing reaction can be accelerated.
In one preferred form of the exhaust gas purifying apparatus as described just above, the temperature increasing means may consist of exhaust gas temperature increasing means for increasing the temperature of exhaust gases discharged from the engine into the exhaust pipe. In this apparatus, the temperature of the NOx catalyst may be increased without using a heater for heating a catalyst.
In another preferred form of the exhaust gas purifying apparatus as described above, the temperature increasing means may consist of ignition timing control means for retarding the ignition timing of the internal combustion engine. In this apparatus, the temperature of the exhaust gases, and consequently that of the NOx catalyst, can be increased by retarding the ignition timing of the internal combustion engine.
The exhaust gas purifying apparatus may be installed in an in-cylinder injection type internal combustion engine in which a fuel is directly injected into each combustion chamber of the engine. In this case, the exhaust gas temperature increasing means may consist of fuel injection control means for conducting fuel injection with respect to each cylinder of the engine a plurality of times during one combustion cycle of each cylinder.
In the apparatus described just above, the fuel can be injected into each cylinder of the engine in suitable timing a plurality of times during one combustion cycle, so that the temperature of the exhaust gases, and consequently, that of the NOx catalyst, can be increased.
The exhaust gas purifying apparatus as described above may further include air-fuel ratio control mans for controlling the air-fuel ratio of the internal combustion engine so as to provide a fuel-rich air/fuel mixture. With this arrangement, the engine operates in a rich-burn combustion mode, so that a reducing atmosphere is suitably formed around the NOx catalyst, thereby to induce reactions needed to resume the purifying capability of the NOx catalyst.